1. Field of the invention
The present invention relates to a stabilized thermoplastic resin composition which is reinforced with fibrous magnesium oxysulfate (abbreviated to MOS hereinafter).
2. Description of prior art
It is well known to incorporate inorganic materials into thermoplastic resins for the purpose of improving various properties of the thermoplastic resins, such as rigidity, mechanical strength, heat resistance, molding shrinkage and dimensional stability. Examples of the inorganic materials employable for that purpose include particulate substances such as calcium carbonate, barium sulfate and magnesium hydroxide; plate-shaped or flake-shaped substances such as talc and mica; and fibrous substances such as glass fiber and asbestos. The particulate substances, among these inorganic materials, hardly give high reinforcing effect to the thermoplastic resins, so that the thermoplastic resin molds obtained by adding the particulate substances to the resins cannot be always employed for industrial materials requiring high characteristics. On the other hand, the plate-shaped or flake-shaped substances and the fibrous substances show high reinforcing effect in one or two-dimensional directions, so that these substances are widely used as reinforcements for thermoplastic resins. However, since the above-mentioned reinforcements showing high reinforcing effect also have various drawbacks, they are sometimes restricted in the use thereof depending on the purpose. For example, products obtained by molding the thermoplastic resins containing the plate-shaped or flake-shaped substances, are apt to have flow marks occurring in the molding procedure and generally show lower-rigidity as compared with molded products obtained from the thermoplastic resins containing fibrous substances such as fiber glass. On the other hand, the thermoplastic resins containing fibrous substances are molded to show poor elongation and to likely have silver streaks on the surfaces of the molded products. Further, the surfaces of the molded products mostly exhibit low glossiness. Particularly in the case of using a glass fiber, the resulting molded product shows low resistance to falling ball impact. Accordingly, in the use of the above-mentioned conventional reinforcements, it is necessary to make an appropriate selection according to the purpose.
The above-mentioned various drawbacks can be removed to a certain extent by appropriately selecting or modifying base materials, subjecting inorganic materials to be used as the reinforcements to surface treatment, adding other appropriate additives, adjusting molding and processing conditions, etc., but satisfactory removal of the drawback can be hardly obtained.
As a thermoplastic resin composition which is improved in various drawbacks found in conventional thermoplastic resin compositions containing the above-mentioned reinforcements, a fiber-reinforced thermoplastic resin composition comprising a thermoplastic resin and fibrous magnesium oxysulfate (MOS) have been proposed. A composition comprising polypropylene and MOS is, for example, disclosed in Japanese Patent Provisional Publication No. 57(1982) 109846, and a kneaded composition of a thermoplastic resin and MOS is disclosed in Japanese Patent Provisional Publication No. 59(1984) 172533.
MOS releases water of crystallization at approx. 250 .degree. C. , whereby a surface of the resulting molded product becomes unstable likely to show silver streaks. A composition improved in this point is disclosed in Japanese Patent Provisional Publication No. 61(1986).72038, and this composition comprises polypropylene, MOS and calcium oxide. However, calcium oxide needs to be handled or stored very carefully, because it rapidly reacts with water and is unstable in air.